Synthetic or man-made diamonds, manufactured by HPHT (high pressure high temperature), CVD (chemical vapour deposition) or other industrial, non-geological processes, have a wide variety of industrial applications, but currently form only a small percentage of the gemstone industry. Being man-made, they do not attract the high values associated with natural diamonds of similar colour and quality and it is clearly desirable from a consumer perspective to provide reliable means of identifying and separating synthetic diamonds from natural ones.
Treated diamonds are natural diamonds which have been artificially enhanced to improve their physical characteristics, usually in terms of their colour or quality. Colour changes can be induced via treatments such as the application of coatings, irradiation and heating. Heating at high temperatures can lead to the conversion of diamond to graphite and this is avoided by applying a stabilising pressure during heating, so called High Pressure High Temperature (HPHT) treatment. Quality can be improved by the application of treatments such as the filling of cracks to reduce their visibility and the removal of inclusions using laser drilling. Diamonds treated in such ways are also considered to be of lower value than the equivalent diamond that has not been subjected to treatment and detection techniques for such treatments are an essential part of ensuring that the purchaser of a diamond can make a fully informed decision about their purchase.
The task of identifying the origin of a gemstone typically falls to a diamond appraiser, grader or gemmologist in the course of preparing a grading certificate or appraisal. The origin of a diamond is a key factor in its market value and is of paramount importance to the gemmologist. There are numerous characteristics that can be used to distinguish between the diamond from nature and one produced from an industrial process (which may be called a synthetic) but the inherent variability in the natural diamond and of the synthetic processes makes such a task difficult and onerous.
One characteristic that has proven to be of utility is the emission of luminescence when a diamond is illuminated (or excited) by a source of energy, most commonly but not exclusively, electromagnetic radiation. A gemmologist would normally have an ultraviolet lamp, perhaps emitting radiation with a wavelength of 365 nm or 254 nm (nanometers), these being common lines in the emission of the low pressure mercury lamp, and might observe what would be called fluorescence. Fluorescence is a type of luminescence characterised as only being produced when the ultraviolet excitation is on. Phosphorescence, which may also be observed, is a type of luminescence that remains but decays away once the excitation is removed. Through interpretation of any such luminescence present, taking into account their observable temporal characteristics, colours and spatial distribution inferences on the task at hand may be drawn as is known in the art.
The DiamondView® as disclosed in U.S. Pat. No. 5,883,389 allows a more sophisticated observation to be made. In particular it offers a source of shorter wave ultraviolet radiation (characterised in having a wavelengths of less than 225 nm) corresponding to the primary absorption edge and only penetrating a very small amount (about 1 μm) into the surface of the diamond so that one could consider that any observed luminescence is produced at the surface. The instrument as disclosed may also incorporate a sensitive camera so that images may be recorded of the observed luminescence and phosphorescence but this is not an essential feature.
A competent gemmologist will know that the terms fluorescence and phosphorescence, while convenient, are merely a loose way of describing the temporal characteristics of luminescence. They describe emissions that decay away quickly or slowly on a time scale of human observation. Not surprisingly the temporal characteristics of luminescence are far more complex. It is known for example that, if subjected to a hypothetically very short pulse of excitation, luminescence may be observed to decay on time scales from picoseconds to tens of seconds. There are also multiple possible decay laws, such as an exponential decay or a power law decay, depending on the kinetics of the underlying radiative and competing non-radiative processes. Furthermore, a sample may show a combination of emission colours or wavelengths and temporal characteristics in each of a plurality of locations.
Said gemmologist might also know that a fuller understanding of said temporal, spatial and spectral characteristics would be advantageous to the task at hand, but lacks a convenient apparatus and method to perform the required observations in a practical manner in a reasonable time and at economic cost beyond what can in essence be performed by eye.